I sometimes use the histogram on my camera to double-check my exposure of model's face when shooting portraits. My technique doesn't work in all situations with on-camera flash but does with off-camera flash and ambiant lighting.
I take a very close shot of just the models face and hair (making sure not to include much, if any background). The histogram should then not touch either the left (dark) or right (light) side of the display. If it does it indicates that I've got a bad exposure.
You can also do a similar thing in photoshop, by doing a marquee selection of just the face and hair and then looking at the Levels. The levels shown will be of just the selected area. See below.
Give it a try and see if it helps.
Mark Oehler